1. Field of the Invention
The present invention relates to an electric pump, and more particularly, to an electric pump mounted on a vehicle and used for supplying or circulating, for example, cooling water, fuel, or oil.
2. Description of the Related Art
Conventionally, a pump mounted on a vehicle is driven through transmission of rotation of a pulley attached to a rotary shaft of an engine, to a pulley attached to a rotary shaft of the pump via a belt. Since the pump is driven via the belt, the pump must be installed in an extremely limited location near the engine. Since many components, including the engine, are disposed within the small space of the engine compartment, restrictions are imposed on the shape of the pump and the installation space therefor. Therefore, there has been a problem in that the restricted arrangement of the pump within the engine room lowers the degree of freedom in design.
Further, a mechanical pump, which has conventionally been used as a pump for vehicles, employs a shaft seal structure. That is, such a mechanical pump requires a shaft seal, which causes problems such as generation of mechanical loss, leakage, and short service life.
In view of such problems, there has been proposed use of an electric pump in which, in place of an engine, an electric motor is used as a drive source. In one example configuration, the shaft of an impeller is rotated by means of an electric motor, and an O-ring seal is provided in order to realize a fluid-tight structure for the shaft. In another configuration, a magnetic coupling is interposed between the shaft of an impeller and the rotary shaft of an electric motor. However, these configurations bring about problems such as an increase in the overall length, necessity of secure maintenance of seal members, necessity of securing following operation of the coupling, and necessity of additional components such as a cover.
In such an electric pump, a brush type electric motor or an outer-rotor type electric motor is employed. The brush type electric motor inherently has the problem of wear of brushes and generation of sliding noise. The outer-rotor type electric motor has a problem in that when it is configured for use in water by being provided with submerged bearings, the area of contact between the rotor and fluid increases, with a resultant increase in loss caused by fluid (e.g., water loss).
In view of the foregoing, there has been developed an electric pump which can secure fluid tightness and decrease the number of components. This electric pump includes rotor magnets fixedly disposed at uniform angular intervals around a shaft, which is common with an impeller; a casing having a portion surrounding the rotor magnets, wherein at least that portion is formed of a non-magnetic material; and a stator disposed to surround the rotor magnets via the casing and adapted to generate a rotating magnetic field (see Japanese Patent Application Laid-Open (kokai) No. H5-010286. FIG. 5 shows a cross sectional view of the conventional electric pump.
Since the stator is isolated from the rotor magnets of the pump by means of the casing, the fluid tightness of the pump can be reliably maintained. Moreover, since the stator, by its very nature, does not have a rotating portion, its operation is not adversely affected by debris or dust.
As shown in FIG. 5, a hub 104 is fixed to a shaft 103 of an impeller 102; and a plurality of permanent magnets; i.e., rotor magnets 105, disposed at uniform angular intervals about the shaft 103 are fixed to the periphery of the hub 104. The shaft 103 is rotatably supported by means of bearings 111 and 112 fixedly disposed within a casing 101.
Meanwhile, the stator is provided so as to surround the rotor magnets 105 via the casing 101. The stator includes a core 106, and a coil 107 wound around the magnetic poles of the core 106. At least a portion of the casing 101 sandwiched between the rotor magnets 105 and the stator is formed of a non-magnetic material such as resin in order to prevent induced current from flowing.
In the electric pump shown in FIG. 5, the thickness of the casing 101 must be reduced in order to produce a strong magnetic attractive/repelling force between the rotor magnets 105, and the core 106 having the coil 107 wound thereon. However, the casing 101 must have sufficient strength (e.g., thickness) for attachment of the core 106 having the coil 107 and journalling of the rotor. Moreover, the casing 101 must have a thickness greater than a predetermined value in order to resist water pressure generated by means of the impeller 102. Moreover, since at least a portion of the casing 101 must be formed of a non-magnetic material such as resin, reducing the thickness of the casing is difficult. Therefore, a restriction is imposed on the magnetic coupling action between the rotor magnets 105 and the core 106 having the coil 107, and consequently the formed electric motor has poor rotation characteristics. Further, the bearing 111 produces an irregular flow of fluid; i.e., generates vortexes, thereby lowering the efficiency of the pump. The electric motor is of an AC type.